This invention relates to a method for the surface treatment of aluminum or an aluminum alloy. More particularly, it relates to a method for the surface treatment of aluminum or an aluminum alloy wherein a film is formed thereon by a chemical reaction, without supplying energy (e.g., electricity) externally.
The conventionally employed alumite process is a method for forming a hard film of aluminum oxide on the surface of aluminum by oxidizing aluminum anodically in an acidic bath. However, this method has the disadvantage of involving a high cost because it requires equipment for electric supply and the rate of film formation is slow.
Now, there has been developed a technique for forming a film on the surface of aluminum by heating an aqueous solution containing magnesium silicofluoride and ammonium silicofluoride to a temperature of 70 to 100° C. and soaking aluminum in this aqueous solution (see Japanese Patent Provisional Publication No. 11-193478/'99). This technique makes it possible to simplify the equipment and reduce treating costs and also to form a film having excellent sliding properties on the surface of aluminum.
However, the aforesaid technique shows a tendency in which the thickness of the film formed on the surface of aluminum first treated after bath preparation is small and, subsequently, the film thickness becomes larger as the treated area of aluminum increases. This causes the problem that there is great variation in film thickness among aluminum products.